Most hydrogen is supplied from steam reforming of hydrocarbons, more particularly methane (SMR). The reformed gas is generally sent to a shift reactor (water-gas shift reactor) to produce more hydrogen. The water-gas shift reaction is a reaction between carbon monoxide and water in order to form carbon dioxide and water.
The gas produced generally has the following characteristics:                pressure from 15 to 40 bar abs;        temperature close to the ambient temperature (after cooling);        composition in mol %: between 60 and 80% H2; between 15 and 25% CO2; between 0.5 and 5% CO; between 3 and 7% CH4; between 0 and 6% N2, saturated with water;        flow rate: from a few thousand to a few hundred thousand Nm3/h.        
Such a gas is then generally sent directly to a hydrogen PSA (Pressure Swing Adsorption) in order to produce high-purity hydrogen (99% to 99.9999 mol %).
The waste from the PSA contains all the CO2, the great majority of the CH4 and CO, a large portion of the N2 and the hydrogen in an amount that depends on the yield of the PSA unit (from 75 to 90% depending on the desired efficiency).
This waste, with the CO2 contained therein, is burnt in the steam reforming furnace. The waste gas from this unit is vented to atmosphere after recovering some of the available heat.
However, climate change constitutes one of the greatest environmental challenges. The increase in carbon dioxide concentration in the atmosphere is for a very large part the reason for global heating. In the context for reducing CO2 emissions, capture of at least some of the CO2 emitted must be envisaged.
It is known to remove CO2 from such a fluid by scrubbing, for example scrubbing with amines, upstream of the PSA.
The drawback of such a solution is essentially its energy cost inappropriate to the capture problem.
Other solutions are based on adsorption.
Document EP 0 341 879 B1 treats the waste from an H2 PSA in order to extract the CO2 therefrom via a PSA and a refrigeration.
Document WO 2006/054008 also treats the waste from an H2 PSA via a PSA and a cryogenic unit.
Document U.S. Pat. No. 5,026,406 describes the production of two high-purity fractions using a PSA. Example 2 partly corresponds to the problem posed here. A CO2-rich (99.7 mol %) fraction and a fraction containing about 90 mol % 147 are obtained. In practice, this fraction has to be treated in a second unit, for example a PSA of the H2 PSA type, in order to obtain an H2 purity of 99%. It is necessary to use vacuum pumps and/or recycling in order to achieve the intended performance.
Document US 2007/0227353 also relates to the same technical problem. The recommended solution is again the use of a PVSA, i.e. an adsorption unit with vacuum steps. It is known that such steps, although they are efficient in terms of performance, are expensive in terms of capital investment (vacuum pumps) and in terms of energy.
Moreover, document FR 2 884 304 describes an adsorption unit operating at a maximum pressure of 10 bar absolute, producing a CO2-enriched gas which is sent to a cryogenic unit that enriches this gas up to a minimum of 80 mol %. The CO2-depeleted gas from the adsorption unit is expanded, after being cooled or not, in order to supply the refrigerating power of the cryogenic unit. At least one portion of the CO2-lean gas from the cryogenic unit is recycled to the PSA, after being expanded or not. At least one portion of the CO2-lean gas from the cryogenic unit is used as fuel. Said adsorption unit may be a VSA, a VPSA or a PSA.
The process and/or plant envisaged in document FR 2 884 304 comprises only a single adsorption unit.
That document essentially based on recovery of CO2 in a fluid at a pressure below 10 bar abs does not relate to the associated production of hydrogen. In particular, it does not relate to SMR output gases.
The use of a PSA as described in FR 2 884 304 is not effectively suitable a priori when it is desired to extract CO2 from a gas at a high pressure, such as syngas, whenever it is also desired to recover as a priority most of one of the least-adsorbable constituents. Specifically, since CO2 is an easily adsorbable constituent, a moderate partial pressure, of the order of one bar, is sufficient for obtaining quasi-saturation of the adsorbents conventionally used, such as zeolites or active carbons. The use of a high pressure therefore provides nothing to the arresting of CO2 but is unfavorable when it is desired to keep constituents, such as hydrogen, under pressure. Since the latter constituent is very weakly adsorbed, it is most particularly present in the adsorber in the gas phases, whether in the pore volume of the adsorbent, in the intergranular volume or in the in-out dead volumes. For a given volume, the loss of hydrogen is therefore proportional to the pressure. In such an application, it is normal to envisage instead to treat the waste from the H2 PSA in order to sequester the CO2 knowing that this is a low-pressure stream enriched in CO2 and depleted in hydrogen. WO 2006/054008 and EP 0 341 879 B1 are based on these considerations.
One solution envisaged for partially obviating these drawbacks is, as mentioned, the use of a complex PSA cycle employing a vacuum to extract the CO2. Under these conditions, the adsorbent may be used efficiently and the use of internal recycling then enables the loss of hydrogen to be limited. This pays for itself in terms of capital investment and energy consumption.
Starting from this situation, one problem that arises is how to provide a process capable of economically producing hydrogen with CO2 capture and with no appreciable loss of hydrogen.